The invention concerns an apparatus with devices comprising glass for the polarization of noble gases in the sample cell. The invention further concerns a method of operating the apparatus.
Recent developments in magnetic resonance tomography (MRT) and in magnetic resonance spectroscopy (NMR) with polarized noble gases lead to the expectation of uses in medicine, physics and material sciences. High levels of polarization of nuclear spins of noble gases can be achieved by optical pumping by means of alkali metal atoms, as can be seen from the publication Happer et al, Phys. Ref. A, 29, 3092 (1984). Typically at the present time the alkali metal atom rubidium is used in the presence of a noble gas and nitrogen. It is possible in that way to achieve nuclear spin polarization of the noble gas xenon (129Xe) of about 20 percent. Such a level of nuclear spin polarization is about 100,000 times greater than the equilibrium polarization in clinical magnetic resonance tomographs. The drastic increase in the signal-noise ratio, which this involves, explains why in future new possible uses are expected in medicine, science and technology.
The term polarization is used to denote the degree of orientation (order) of the spins of atomic nuclei or electrons. 100 percent polarization means for example that all nuclei or electrons are oriented in the same fashion. The polarization of nuclei or electrons involves a magnetic moment.
Polarized xenon is for example inhaled by or injected into a human being. Between 10 and 15 seconds later the polarized xenon collects in the brain. The distribution of the noble gas in the brain is detected by means of magnetic resonance tomography. The result is used for further analysis procedures.
The choice of the noble gas depends on the situation of use. 129Xenon involves a large chemical shift. If xenon is adsorbed for example on a surface its resonance frequency significantly changes. In addition xenon is dissolved in fat-loving (that is to say: lipophilic) liquids. If properties of that kind are wanted, xenon is used.
The noble gas helium scarcely dissolves in liquids. The isotope 3helium is therefore used regularly when cavities are involved. The lung of a human being represents an example of such a cavity.
Some noble gases have valuable properties other than those referred to above. Thus for example the isotopes 83krypton, 21neon and 131xenon have a quadrupole moment which is of interest for example in experiments in basic research or surface physics. Those noble gases however are very expensive so that they are not suitable for uses in which relatively large amounts are used.
It is known from the publication xe2x80x9cB. Driehuys et al., Appl. Phys. Lett. 69, 1668 (1996)xe2x80x9d for noble gases to be polarized in the following manner in a polarizer.
Circularly polarized light is provided by means of a laser, that is to say light in which the rotational impulse or spin of the photons are all in the same direction. The rotational pulse of the photons is transmitted to free electrons of alkali metal atoms. The spins of the electrons of the alkali metal atoms therefore have a great deviation from thermal equilibrium. The alkali metal atoms are consequently polarized. By a collision of an alkali metal atom with an atom of a noble gas, the polarization of the electron spin is transferred from the alkali metal atom to the atom of the noble gas. That results in polarized noble gas.
Alkali metal atoms are used as they have a high optical dipole moment which interacts with the light. In addition alkali metal atoms each have a free electron so that no detrimental interactions can occur between two and more electrons per atom.
Cesium would be a particularly well-suited alkali metal atom which is superior in relation to rubidium to achieve the above-indicated effects. At the present time however there are no lasers available with a sufficiently high level of power, as would be required for the polarization of xenon by means of cesium. There is however the expectation that in future lasers with power levels of around 100 watts on the cesium wavelength will be developed. In that case cesium will probably be preferably used for the polarization of xenon gas.
In accordance with the state of the art, a gas mixture is passed slowly under a pressure of typically between 7 and 10 bars through a cylindrical glass cell. The gas mixture comprises for example 98 percent of 4helium, one percent of nitrogen and one percent of xenon. The typical speeds of the gas mixture are some ccm per second.
The gas mixture firstly flows through a vessel (hereinafter referred to as the xe2x80x9csupply vesselxe2x80x9d) in which there is about one gram of rubidium. The supply vessel with the rubidium therein is heated together with the adjoining glass cell to between about 100 and 150 degrees Celsius. The rubidium is evaporated by the provision of those temperatures. The concentration of the evaporated rubidium atoms in the gas phase is determined by the temperature in the supply vessel. The gas flow transports the evaporated rubidium atoms from the supply vessel into the cylindrical sample cell. A powerful, circularly polarized laser (100 watt output in continuous operation) irradiates the sample cell which is generally a cylindrical glass cell, axially, and optically pumps the rubidium atoms into a highly polarized condition. In this case the wavelength of the laser must be matched to the optical absorption line of the rubidium atoms (D 1xe2x80x94line). In other words: in order to optimally transfer the polarization of light to an alkali metal atom, the frequency of the light must coincide with the resonance frequency of the optical transition. The sample cell is in a static magnetic field B0 of a few 10 Gauss, which is produced by coilsxe2x80x94in particular a so-called Helmholtz coil pair. The direction of the magnetic field extends parallel to the axis of the cylindrical configuration of the sample cell or parallel to the laser beam direction. The magnetic field serves to guide the polarized atoms.
The rubidium atoms which are optically highly polarized by the light of the laser collide in the glass cell inter alia with the xenon atoms and deliver their high level of polarization to the xenon atoms. At the outlet from the sample cell the rubidium is deposited at the wall by virtue of the high melting point in comparison with the melting points of the other gases. The polarized xenon or the residual gas mixture is conducted from the sample cell into a freezing-out unit. This comprises a glass flask whose end is immersed in liquid nitrogen. The glass flask is further disposed in a magnetic field of a strength of between 1000 and 2000 Gauss. The highly polarized xenon gas is deposited in the form of ice at the inside glass wall of the freezing unit. At the outlet from the freezing unit the remaining gas (helium and nitrogen) is passed by way of a needle valve and finally discharged.
The flow speed in the entire arrangement can be controlled by way of the needle valve and measured with a measuring device. If the flow speed rises excessively greatly, there is no time remaining for transfer of the polarization effect from the rubidium atoms to the xenon atoms. No polarization is therefore achieved. If the flow speed is too low, too much time elapses before the desired amount of highly polarized xenon has frozen. More specifically due to relaxation the polarization of the xenon atoms decreases again. Relaxation of the xenon atoms is greatly slowed down by the freezing effect and due to the strong magnetic field to which the freezing unit is exposed. It is therefore necessary for the noble gas xenon to be frozen after the polarization operation as quickly as possible and without loss. Admittedly, it is not possible to avoid relaxation, by virtue of the freezing procedure. However, between 1 and 2 hours still remain before xenon polarization has fallen so greatly that further use of the initially highly polarized gas is no longer possible.
A polarizer of the above-indicated kind always has connecting locations. Connecting locations are those at which at least two conduits through which polarized gas is conducted are connected together. The conduits generally comprise glass. The connection is made by a connecting element such as for example flanges.
The light of the laser which produces the polarization effect is absorbed in the sample cell. The intensity of the light and thus the degree of polarization of the alkali metal atoms in the sample cell correspondingly decreases. For technical reasons, the cross-section of the sample cell is generally not uniformly stimulated by the light of the laser. Consequently the alkali metal atoms are not uniformly polarized. Interactions with the walls of the sample cell also change polarization of the alkali metal atoms along the cross-section of the sample cell. Consequently the polarization of the alkali metal atoms changes in the sample cell, in dependence on the location therein.
In order to polarize a single free alkali metal atom a certain amount of energy is required. That required energy corresponds to the resonance frequency for raising the free electron of the alkali metal atom from a ground state to an excited state. In order to optimally transfer the energy from a laser to the alkali metal atom, the frequency of the light of the laser must be tuned to the resonance frequency of the alkali metal atom. Inexpensive lasers emit their light within a given frequency spectrum. This therefore does not involve an individual frequency but a distribution of frequencies. The available spectrum of a laser is characterized by a so-called line width. In order to polarize alkali metal atoms economically, lasers are provided, whose frequency and line width are tuned to the resonance frequency and optical line width respectively of the alkali metal atom.
In order to be able to better transfer the energy from a laser to alkali metal atoms, collision partners are provided for the alkali metal atoms during polarization. In particular 4helium atoms serve as collision partners. Due to the interaction or the collisions with the helium atoms, the optical line width of an alkali metal atom is widened. The wider that atomic spectrum is, the correspondingly more is it possible to use spectrally wide and thus inexpensive lasers.
The number of collisions between an alkali metal atom and a collision partner such as helium increases, in proportion to increasing pressure. For 4helium for example the pressure broadening of the optical line width of the alkali metal atom is proportional to the pressure of the helium gas. In addition helium enjoys the valuable property that it has only a little disturbing influence on polarization of the alkali metal atoms. Therefore, in the polarization of alkali metal atoms by a laser, operation is usually implemented with a gas mixture which comprises 98 percent helium and is at a pressure of about 10 bars.
The 100 watt laser used in accordance with the state of the art involves a glass fiber-coupled diode laser with a typical spectral width of between 2 and 4 nanometers. With a gas pressure of 10 bars the line width of the optical transition of rubidium atoms is widened to about 0.3 nanometer. Therefore, in the present rubidium-xenon polarizers in which expensive diode lasers with typically a 2 nanometer line width are used for the optical pumping procedure, only a fraction of the laser light is put to use.
The partial pressures of helium are at the present time up to 10 bars in a gas mixture in accordance with the state of the art. That is very high in comparison with the other partial pressures (xenon and nitrogen respectively). That relatively high partial pressure means that polarized atoms only rarely reach the sample wall of the glass cell and there lose their polarization for example due to interaction with paramagnetic centers. With increasing helium partial pressure, there is a decreasing probability of polarized atoms disadvantageously impacting against the cell wall.
In order to use the full laser power and at the same time to reduce disadvantageous relaxation effects due to collisions with the wall, operation had to be implemented at helium pressures far above 30 bars. That is not possible in the state of the art.
In terms of the composition of the gas mixture, the following is also to be borne in mind.
A polarized alkali metal atom such as for example rubidium can produce fluorescence radiation. If such radiation is captured by a further polarized alkali metal atom that capture results in depolarization of the alkali metal atom. The nitrogen in the gas mixture, which is used in the polarization of noble gases, serves for capture of that fluorescence radiation in order to reduce the above-mentioned unwanted depolarization effect. The element nitrogen in the gas mixture, like xenon, has only a low partial pressure. That partial pressure is typically 0.1 bar in the state of the art.
The heavy noble gas atoms such as for example xenon atoms, upon collisions with the alkali metal atoms, cause severe relaxation of the polarization of the alkali metal atoms. In order to keep polarization of the alkali metal atoms as high as possible in the optical pumping procedure, the partial pressure of the xenon gas in the gas mixture must be correspondingly low. Even with a xenon partial pressure in the gas mixture of 0.1 bar, laser outputs of around 100 watts are used in order to achieve polarization of the alkali metal atoms of 70 percent in the entire sample volume.
The state of the art involves using glass cells which are blown from one piece of glass. It was hitherto not possible to produce a glass cell, in some other fashion, which could withstand the desired high pressures. The consequence of the above-indicated manner of manufacture of the glass cell is that the windows through which the laser light enters and exits are always curved or rounded. Unwanted disadvantageous lens effects occur in the entry or exit of the laser light. The light of the laser is focused or expanded. That causes a considerable worsening in terms of the effectiveness of polarizing alkali metal atoms in the gas mixture of the glass cell.
A glass cell for the polarization of noble gases should satisfy the following demands:
It should be non-magnetic or resistant in relation to alkali metals at temperatures of up to 200 degrees Celsius and pressure-resistant.
It should be possible to close the glass cell with valves. The valve heads should withstand 200 degrees Celsius in the presence of the gas mixture, in addition they should be non-magnetic and pressure-resistant. The influence of the valves on polarization of the noble gas should be as low as possible.
The surface in the interior of the cell should have no disturbing influence on xenon or on rubidium polarization. There should therefore be no para- or indeed ferromagnetic centers at the inside wall of the cell. The material of which the cell is made up should also be absolutely non-magnetic.
It should be possible for the light of the laser to be propagated through the cell as far as possible without any lens effects, that is to say parallel.
The entry window of the cell should absorb the light of the laser as little as possible. Otherwise the window is excessively heated and finally destroyed.
The entry window should not be birefringent either at normal pressure or at high pressure. Otherwise circular polarization of the laser would be destroyed or at least reduced.
Overall therefore it is desirable for a noble gas to be polarized at pressures of far above 30 bars. The sample cell which comprises glass should be subjected to the minimum possible loading, due to the high pressures.
The object of the invention is to provide a polarizer for noble gases, in which broad-band and narrow-band lasers can be equally used in the optimum fashion.